The present invention relates to apparatus and methods for aligning openings through wheels and spacers during assembly to form a turbine rotor stack enabling subsequent insertion of steam tubes into the aligned openings with the tightest possible clearances between the openings and tubes and particularly relates to a fixture having alignment pins shaped to enable tangential alignment of the elements of the rotor stack without constricting radial alignment.
Gas turbine rotors are typically formed by stacking the rotor wheels and spacers axially one against the other. Bolt holes are provided through the wheels and spacers and receive bolts which are used to finally secure the wheels and spacers to one another to form the rotor. The wheels and spacers in final assembly also have rabbeted joints. That is, axially projecting flanges formed on the spacers underlie and fit tightly against axially oppositely extending flanges formed on the wheels. To form the rabbeted joints, the wheels are typically heated in an oven prior to assembly in the stack to expand the flanges of the wheels so that, after stacking and upon cool-down, the flanges of adjacent wheels and spacers fit tightly relative to one another.
During stack-up of the wheels and spacers, the bolt holes of the wheels and spacers are fitted over bolts projecting from a fixture. The bolts remain in the rotor assembly and maintain the wheels and spacers stacked relative to one another. Consequently, to enable the stack-up of the wheels and spacers on the bolts, substantial clearances between the bolt holes through the wheels and spacers and the bolts are necessary in the radial direction and corresponding clearances are therefore also provided in the circumferential direction. A need has developed, however, for a much tighter alignment of the stacked wheels and spacers which cannot be provided by the alignment of the bolt holes and bolts during the assembly stack-up consistent with the need to accommodate radial expansion and contraction of the heated wheels during the stack-up.
This need has arisen as a result of a new advanced steam-cooled gas turbine design of the assignee of the present invention wherein certain parts of the rotor are steam-cooled. In this advanced steam-cooled turbine design, a plurality of openings, in addition to the bolt holes, are provided through the wheels and spacers of the rotor to accommodate a plurality of circumferentially spaced tubes extending generally axially through the rotor for supplying steam to the steam-cooled parts, i.e., first and second stage rotor buckets, and returning the spent cooling steam to the rotor bore assembly. The supply and return tubes are thin-walled structures which extend through openings in bushings provided in circumferentially spaced apertures of the stacked wheels and spacers. Tight clearances between the tubes and bushing openings are highly desirable. The steam-carrying tubes desirably have as large a diameter as possible to maximize steam flow, as well as have very tight clearances with the openings to preclude high stresses on the tubes. Thus, there is a need to tightly tolerance the openings through the wheels and spacers which carry the steam-cooling tubes while concurrently enabling radial contraction of the wheels to form tightly rabbeted joints.
In accordance with the present invention, a plurality of alignment pins are employed in a fixture for reception in the openings of the wheels and spacers. The alignment pins are specifically configured to allow for radial misalignment of the openings due to thermal expansion and contraction, tolerance stack-up and wheel-to-spacer mismatch due to rabbet mechanical growth. The alignment pins, however, enable closely toleranced circumferential alignment of the wheels, spacers and aft shaft bushing openings sufficiently to install the steam-carrying tubes into the aligned openings after the rotor has been constructed with a tight clearance and minimal stress in use. The configuration of the alignment pins allows tight tangential alignment of the wheels and spacers without constricting radial alignment thereof and enables the pins to find the average position of all the openings so that the most each opening can be off in a circumferential direction is the opening""s tolerance relative to the average true circumferential position of each pin. To accomplish this, the alignment pins have a radial dimension less than their dimension in the circumferential direction. Preferably, the pins are generally hexagonal in cross-sectional configuration with major and minor axes extending in circumferential and radial directions.
To accomplish the foregoing, the rotor fixture comprises a stand having a plurality of precision-located alignment openings which receive alignment pins circumferentially spaced from one another. The pins upstand from the fixture. Each pin has a cross-section with a radial dimension substantially less than its circumferential dimension to accommodate radial expansion and contraction during assembly of the rotor, while at the same time affording alignment of the wheels and spacers so that the average position of all the aligned openings is equal to or less than the tolerance of the openings relative to the average true position of each pin.
To form the stack in accordance with the present invention, the bolts are also provided on the fixture and upstand the full length of the stack. The height of the alignment pins above the fixture is adjustable so that the pins can be periodically raised as the wheels and spacers are stacked one on top of the other. (The following description proceeds with stacking four wheels and three spacers on an aft shaft to form a four-stage rotor, with the aft wheel being designated the fourth wheel and the forward wheel the first wheel, it being appreciated that the stacking method hereof can be applied to rotors having different numbers of wheels and spacers and hence a different number of stages.) To begin the rotor assembly, the aft shaft including the integral aft shaft disk is disposed on the fixture with the bolts being received through bolt apertures on the aft shaft disk and the alignment pins being received through slave bushings on the aft shaft disk. With the aft shaft on the fixture, an initial wheel, e.g., the fourth wheel, is heated in an oven. Once the fourth wheel is heated, it is placed on the aft shaft disk with the bolts and alignment pins being received through its bolt holes and openings, respectively. By initially heating the fourth wheel, the forwardly directed flange of the aft shaft disk lies radially inwardly of the now radially expanded, axially directed, aft flange of the fourth wheel. While the fourth wheel remains heated, the 3-4 spacer is then applied to the fixture with the bolts being received in the bolt holes of the spacer and the alignment pins being received in the spacer openings. The aft-directed flange of the 3-4 spacer is received radially within the radially expanded forwardly directed flange of the fourth wheel. The fourth wheel is then allowed to cool. Consequently, the aft flange of the fourth wheel tightly engages the forward flange of the aft shaft disk and the forward flange of the fourth wheel engages the aft flange of the 3-4 spacer to form tight rabbeted joints. It will be appreciated that the fourth wheel contracts radially as it is allowed to cool down and engage the corresponding flanges.
The alignment pins are then elevated in the fixture to receive the next wheel/spacer set, i.e., the third wheel and the 2-3 spacer. The third wheel is first heated and applied over the bolts and alignment pins similarly as the fourth wheel and the 34 spacer were applied over the bolts and alignment pins. It will be appreciated that the radial contraction of the third wheel during cool-down enables a tight fit between the flanges of the third wheel and the 2-3 and 3-4 spacers on axially opposite sides of the third wheel to form the rabbeted joints. After cool-down, the alignment pins are again raised relative to the fixture to receive the second wheel and 1-2 spacer. The second wheel is thus heated and the heated second wheel and 1-2 spacer are similarly applied to the bolts and alignment pins. After cool-down of the second wheel forming the tightly engaged rabbeted joints between the second wheel and the 1-2 and 2-3 spacers on axially opposite sides of the second wheel, the first or final wheel is heated, similarly applied to the bolts and alignment pins and allowed to cool down to form the rabbeted joint with the 1-2 spacer.
It will be appreciated that upon cooling of the wheels, the flanges radially contract to form portions of the rabbeted joints. That radial contraction is accommodated by the large clearance between the reduced radial dimension of the alignment pins and the openings through the wheels and spacers. However, because the alignment pins have a circumferential dimension corresponding to the tolerance of each spacer or wheel opening relative to the average true position of each pin, a tight alignment of the wheel and spacer openings in a circumferential direction is achieved.
In a preferred embodiment according to the present invention, there is provided a fixture for forming a turbine rotor having stacked axially aligned wheels and spacers, each of the wheels and spacers having a plurality of circumferentially spaced openings thereabout for alignment with one another, comprising a support having an axis for registration with axes of the aligned wheels and spacers, at least a pair of alignment rods spaced radially from the axis of the support and circumferentially from one another, the rods being located about the support for reception in the openings through the wheels and spacers, each of the rods having a radial dimension less than a circumferential dimension such that spacing between the rods and margins of the openings in a radial direction is greater than spacing between the rods and margins of the openings in a circumferential direction.
In a further preferred embodiment according to the present invention, there is provided a method of stacking a plurality of wheels and spacers forming a rotor for a turbine, the wheels and spacers having a plurality of circumferentially spaced, axially extending openings spaced radially from axes of the wheels and spacers, comprising the steps of providing a support fixture having an axis, disposing a plurality of alignment rods about the fixture in circumferentially spaced relation to one another about and generally parallel to the axis, each rod having a cross-section with a radial dimension less than a horizontal dimension and disposing the wheels and spacers on the support fixture with the alignment rods extending through the openings with clearances between the rods and margins of the openings being greater in a radial direction than in a circumferential direction.